1. Field of the Invention
The present invention relates to a process for the co-preparation of isobutene and ethyl tert-butyl ether (ETBE), and in particular to a method for co-producing isobutene and ethyl tert-butyl ether by catalyzing tert-butanol (TBA) mixture with the ethanol undergoes dehydration and etherification.
2. The Prior Arts
Isobutene is the raw material to manufacture alkylate, MTBE/ETBE, diisobutylene, polyisobutene, methacrylic acid, butylated phenols, etc. There are three sources of isobutene: catalytic cracking, steam cracking, and dehydrogenation of butanes. However, in those processes isobutene is always produced as a mixture of C4s isomers. It is costly to produce high purity isobutene from those mixtures due to their close boiling points.
There are currently three important processes for the production of high purity isobutene: (1) the extraction process using a mixture of C4s isomers to separate isobutene, (2) the dehydration of tert-butanol, and (3) the cracking of MTBE. The expected demand for MTBE/ETBE precludes the third route for isobutene production. MTBE/ETBE is more valuable than TBA due to its huge demand as an additive in gasoline fuel. TBA is an important source of isobutene and its derivatives by dehydration of TBA. In addition, the isobutylene and ETBE may be produced from TBA by reaction with ethanol directly into ETBE.
U.S. Pat. No. 5,292,964 discloses a process for preparing alkyl tertiary butyl ether by sequential reaction. For example, TBA and ethanol are reacted in a first reactor and the effluent is charged to the first distillation column, thus a mixture of ethanol and ETBE is obtained from this column top. The ethanol and ETBE mixture is further reacted with isobutylene in a second reactor to form additional quantities of ETBE. Not only excess isobutylene has to be added to increase the ETBE yield, but also, the first distillation column is operated on the condition of ethanol-to-ETBE molar ratio of 1:2 to avoid by-products water which forms an azeotropic behavior in the column. Therefore, the flexibility in operation of the process is limited.
U.S. Pat. No. 5,527,970 discloses a serious of catalysts to enhance TBA conversions and ETBE selectivities. The catalyst may be an acid resin or a pentasil zeolite. However, the obtained TBA conversions and the ETBE selectivities by a continuous flow reactor are limited under the thermodynamic equilibrium reaction. The patent did not disclose the separation of by-product (water) issues.
U.S. Pat. No. 7,825,282 discloses a two-step reaction process to avoid water to form an azeotrope with unreacted ethanol, including dehydrating TBA to form isobutylene, and then reacting isobutylene with ethanol to form ETBE. The process is inefficient, and unable to overcome the transportation and storage problems of TBA.
On the other hand, following patents are focus on the isobutene production via TBA dehydration. U.S. Pat. No. 4,423,271 discloses a method, which uses ion exchange resin as catalyst to dehydrate aqueous TBA in a fixed bed. U.S. Pat. No. 5,811,620 discloses a TBA dehydration method for producing isobutene via a reactive distillation column packed with fluoride-treated catalyst. A similar method is also disclosed in U.S. Pat. No. 5,849,971. However, no ethanol is fed to the column and ETBE cannot be cogenerated with isobutene. In order to manufacture ETBE, an etherification process is required to follow with these TBA dehydration process.
The kinetics of liquid phase synthesis of ETBE from TBA and ethanol catalyzed by ion exchange resin has been proposed by Yin et al., Int. J. Chem. Kinnet., 1995, 27, 1065-1074. In their report, the TBA and ethanol undergo etherification and dehydration simultaneously over Amberlyst 15 such that ETBE and isobutene can be co-produced.
The above methods are not suitable for industrial applications. For example, TBA at high purity is in solid state if its temperature is below 25° C. The transportation for market trade is inconvenient. Also, these processes should be localized around the PO/TBA (propylene oxide/tert-butanol) process or isobutene hydration process to avoid the TBA transportation problem.